Hereditary tyrosinemia type I (HT1) is an inherited metabolic disease caused by a deficiency in the enzyme, fumarylacetoacetate hydrolase (FAH). Death in infancy generally results from the acute form of HT1, while a chronic form typically results in a teen-aged death due to progressive liver failure, liver cirrhosis, liver cancer, renal dysfunction and neurological crises. The FAH enzyme catalyzes a unique chemical reaction required for the final step in the metabolic degradation of the aromatic amino acids, tyrosine and phenylalanine. The FAH deficiency in HT1 results in the accumulation of toxic compounds that may cause DNA and tissue damage and may interfere with other essential metabolic processes, such as heme biosynthesis. The HT1 is accompanied by genetic variability with at least twenty six different mutations identified in the FAH gene in HT1 patients. The FAH defects are detectable at the FAH mRNA level, at the FAH protein level and at the FAH enzymatic activity level. However, a correlation between the genetic background of HT1 patients and their clinical severity is not clear. The specific aims of this proposal are to study the catalytic mechanism of FAH and to provide an understanding of the molecular basis for HT1. The FAH structure will be studied by X-ray crystallography, circular dichroism and fluorescence spectroscopy, ultracentrifugation and thermodynamic analyses of the FAH folding/unfolding curves. The catalytic mechanism and activity of FAH will be studied using site directed mutagenesis, steady-state kinetics and fungal complementation assays. The long-term objectives of these studies are to establish correlations between the molecular and clinical phenotypes of HT1, to elucidate the mechanism of FAH catalysis, and to provide information that might aid in the development of compounds for modulating the catalytic activity and/or stability of FAH.